fix(mdx-loader): use React.Fragment as fragment factory

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Passing elements wrapped with the short syntax of React.Fragment into a component as its children may cause a page to crash #7391) and the maintainers have approved on my working plan.

Motivation

Regression introduced by #4997

I'm not sure where mdx.Fragment comes from—possibly from MDX v2? But I've simply reverted to the behavior before when we had no pragma.

Test Plan

Test links

Deploy preview: https://deploy-preview-7392--docusaurus-2.netlify.app/tests/pages/markdownPageTests/#details

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 70	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 87	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 811 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	105 kB
website/build/assets/js/main.********.js	615 kB
website/build/index.html	38.8 kB

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